This compiler backend prints the internal OPs of a Perl program's syntax tree in one of several space-efficient text formats suitable for debugging the inner workings of perl or other compiler backends. It can print OPs in the order they appear in the OP tree, in the order they will execute, or in a text approximation to their tree structure, and the format of the information displyed is customizable. Its function is similar to that of perl's -Dx debugging flag or the B::Terse module, but it is more sophisticated and flexible.

The opname may be followed by op-specific information in parentheses (e.g. gvsv(*b)), and by targ information in brackets (e.g. leave[t1]).

Next come the op flags. The common flags are listed below ("OP flags abbreviations"). The private flags follow, separated by a slash. For example, vKP/REFC means that the leave op has public flags OPf_WANT_VOID, OPf_KIDS, and OPf_PARENS, and the private flag OPpREFCOUNTED.

Arguments that don't start with a hyphen are taken to be the names of subroutines to print the OPs of; if no such functions are specified, the main body of the program (outside any subroutines, and not including use'd or require'd files) is printed. Passing BEGIN, CHECK, INIT, or END will cause all of the corresponding special blocks to be printed.

Print OPs in the order they appear in the OP tree (a preorder traversal, starting at the root). The indentation of each OP shows its level in the tree. This mode is the default, so the flag is included simply for completeness.

Print OPs in the order they would normally execute (for the majority of constructs this is a postorder traversal of the tree, ending at the root). In most cases the OP that usually follows a given OP will appear directly below it; alternate paths are shown by indentation. In cases like loops when control jumps out of a linear path, a 'goto' line is generated.

Print OPs in a text approximation of a tree, with the root of the tree at the left and 'left-to-right' order of children transformed into 'top-to-bottom'. Because this mode grows both to the right and down, it isn't suitable for large programs (unless you have a very wide terminal).

Draw the tree with standard ASCII characters like + and |. These don't look as clean as the VT100 characters, but they'll work with almost any terminal (or the horizontal scrolling mode of less(1)) and are suitable for text documentation or email. This is the default.

Print OP sequence numbers in base n. If n is greater than 10, the digit for 11 will be 'a', and so on. If n is greater than 36, the digit for 37 will be 'A', and so on until 62. Values greater than 62 are not currently supported. The default is 36.

Use formatting conventions that emulate the output of B::Terse. The basic mode is almost indistinguishable from the real B::Terse, and the exec mode looks very similar, but is in a more logical order and lacks curly brackets. B::Terse doesn't have a tree mode, so the tree mode is only vaguely reminiscient of B::Terse.

For each general style ('concise', 'terse', 'linenoise', etc.) there are three specifications: one of how OPs should appear in the basic or exec modes, one of how 'goto' lines should appear (these occur in the exec mode only), and one of how nodes should appear in tree mode. Each has the same format, described below. Any text that doesn't match a special pattern is copied verbatim.

The real sequence number of the OP, as a regular number and not adjusted to be relative to the start of the real program. (This will generally be a fairly large number because all of B::Concise is compiled before your program is).

0 OP (aka BASEOP) An OP with no children
1 UNOP An OP with one child
2 BINOP An OP with two children
| LOGOP A control branch OP
@ LISTOP An OP that could have lots of children
/ PMOP An OP with a regular expression
$ SVOP An OP with an SV
" PVOP An OP with a string
{ LOOP An OP that holds pointers for a loop
; COP An OP that marks the start of a statement
# PADOP An OP with a GV on the pad

You can use B::Concise, and call compile() directly, thereby avoiding the compile-only operation of O. For example, you could use the debugger to step through B::Concise::compile() itself.

When doing so, you can alter Concise output by providing new output styles, and optionally by adding callback routines which populate new variables that may be rendered as part of those styles. For all following sections, please review "FORMATTING SPECIFICATIONS".

set_style accepts 3 arguments, and updates the three components of an output style (basic-exec, goto, tree). It has one minor drawback though: it doesn't register the style under a new name, thus you may prefer to use add_style() and/or set_style_standard() instead.

This subroutine accepts a new style name and three style arguments as above, and creates, registers, and selects the newly named style. It is an error to re-add a style; call set_style_standard() to switch between several styles.

If your newly minted styles refer to any #variables, you'll need to define a callback subroutine that will populate (or modify) those variables. They are then available for use in the style you've chosen.

The callbacks are called for each opcode visited by Concise, in the same order as they are added. Each subroutine is passed five parameters.

1. A hashref, containing the variable names and values which are
populated into the report-line for the op
2. the op, as a B<B::OP> object
3. a reference to the format string
4. the formatting (indent) level
5. the selected stylename

To define your own variables, simply add them to the hash, or change existing values if you need to. The level and format are passed in as references to scalars, but it is unlikely that they will need to be changed or even used.

compile accepts options as described above in "OPTIONS", and arguments, which are either coderefs, or subroutine names.

compile() constructs and returns a coderef, which when invoked, scans the optree, and prints the results to STDOUT. Once you have the coderef, you may change the output style; thereafter the coderef renders in the new style.

walk_output lets you change the print destination from STDOUT to another open filehandle, or into a string passed as a ref.

This function (not exported) lets you reset the sequence numbers (note that they're numbered arbitrarily, their goal being to be human readable). Its purpose is mostly to support testing, i.e. to compare the concise output from two identical anonymous subroutines (but different instances). Without the reset, B::Concise, seeing that they're separate optrees, generates different sequence numbers in the output.